Which of these numbers is prime? ${9,\ 35,\ 36,\ 43,\ 49}$
A prime number has exactly two factors: 1 and itself. The factors of 9 are 1, 3, and 9. The factors of 35 are 1, 5, 7, and 35. The factors of 36 are 1, 2, 3, 4, 6, 9, 12, 18, and 36. The factors of 43 are 1 and 43. The factors of 49 are 1, 7, and 49. Thus, 43 is a prime number.